EUV light, e.g., electromagnetic radiation in the EUV spectrum (i.e. having wavelengths of about 5-100 nm) may be useful in photolithography processes to produce extremely small features, e.g. sub-32 nm features, in semiconductor substrates such as silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements, e.g., xenon, lithium or tin, indium, antimony, tellurium, aluminum, etc., with one or more emission line(s) in the EUV spectrum. In one such method, often termed laser produced plasma (“LPP”), a plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the line-emitting element, with a laser beam. Another method involves disposing the line-emitting element between two electrodes. In this method, often termed discharge produced plasma (“DPP”), a plasma can be produced by creating an electrical discharge between the electrodes.
Heretofore, various systems in which a line-emitting element is presented for irradiation/electric discharge have been disclosed. Many diverse forms and states have been attempted, to include, presenting the element in pure form, e.g., pure metal, presenting the element as a compound, e.g., a salt, or as an alloy, e.g. with some other metal, or in a solution, e.g., dissolved in a solvent such as water. Moreover, systems have been disclosed in which the line-emitting substance is presented as a liquid, including relatively volatile liquids, a gas, a vapor and/or a solid, and can be in the form of a droplet, stream, moving tape, aerosol, particles in a liquid stream, particles in a droplet stream, gas jet, etc.
For these processes, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. A typical EUV light source may also include one or more EUV mirrors e.g., a substrate covered with a graded, multi-layer coating such as Mo/Si. One or more of these mirrors are then disposed in the sealed vessel, distanced from the irradiation site, and oriented to direct EUV light emitted from the plasma to an EUV light source output. In general, these EUV mirrors may be either near-normal incidence type mirrors or grazing incidence type mirrors. By way of example, for an LPP setup, the mirror may be in the form of an ellipsoidal, e.g. a prolate spheroid having a circular cross section normal to a line passing through its loci near-normal incidence type, with an aperture to allow the laser light to pass through and reach the irradiation site. With this arrangement, the irradiation site may be positioned at or near a first focus of the ellipsoid and the light source output may be positioned at, near or downstream of the second ellipsoid focus.
In addition to generating EUV radiation, these plasma processes described above may also generate undesirable by-products in the plasma chamber which can include out-of-band radiation, high energy ions and debris, e.g., atoms and/or clumps/micro-droplets of the target material. These plasma formation by-products can potentially heat, damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, collector mirrors including multi-layer mirror coatings (MLM's) capable of EUV reflection at near-normal incidence and/or grazing incidence, the surfaces of metrology detectors, windows used to image the plasma formation process, and the laser input window. The heat, high energy ions and/or debris may be damaging to the optical elements in a number of ways, including coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding, roughening or eroding them and/or diffusing into them.
Accessing contaminated or damaged optical elements in the plasma chamber for the purpose of cleaning or replacing the elements can be expensive, labor intensive and time-consuming. In particular, these systems typically require a rather complicated and time consuming purging and vacuum pump-down of the plasma chamber prior to a re-start after the plasma chamber has been opened. This lengthy process can adversely affect production schedules and decrease the overall efficiency of light sources for which it is typically desirable to operate with little or no downtime.
For some target materials, e.g., tin, it may be desirable to introduce an etchant, e.g., HBr or some other halogen-containing compound, or H radicals, into the plasma chamber to etch material, e.g. debris that has deposited on the optical elements. This etchant may be present during light source operation, during periods of non-operation, or both. It is further contemplated that the affected surfaces of one or more elements may be heated to initiate reaction and/or increase the chemical reaction rate of the etchant and/or to maintain the etching rate at a certain level. For other target materials, e.g., lithium, it may be desirable to heat the affected surfaces where lithium debris has deposited to a temperature sufficient vaporize at least a portion of the deposited material, e.g. a temperature in the range of about 400 to 550 degrees C. to vaporize Li from the shield surface, with or without the use of an etchant.
One way to reduce the influence of debris is to move the collector mirror further away from the irradiation site. This, in turn, implies the use of a larger collector mirror to collect the same amount of light. The performance of a collector mirror, e.g., the ability to accurately direct as much in-band light as possible to, e.g., a focal point, depends of the figure and surface finish, e.g., roughness of the collector. As one might expect, it becomes more and more difficult/expensive to produce a suitable figure and surface finish as the size of the collector mirror grows. For this environment, EUV mirror substrate considerations may include one or more of the following: vacuum compatibility, mechanical strength, e.g. high temperature strength, high thermal conductivity, low thermal expansion, dimensional stability, and ease of producing a suitable figure and finish.
Many factors may affect the in-band output intensity (and angular intensity distribution) from an EUV light source and these factors may change over the lifetime of the light source. For example, in an LPP system, changes in collector reflectivity, target size, laser pulse energy and duration and/or coupling of laser pulse and target material, e.g, as a function of steering and focusing may affect in-band EUV output intensity. Thus, it may be desirable to determine which component/sub-systems are adversely affecting in-band EUV output intensity so that the problem can be remedied. If possible, it may be desirable to diagnose the performance of each component/sub-system while they are in position in the light source (i.e. in-situ) and/or while the EUV light source is operating.
With the above in mind, applicants disclose EUV light source components and methods for producing, using and refurbishing EUV light source components.